1. Field of the Invention
The present invention relates to a fuel cell power generator system, more particularly to a method of and an apparatus for controlling the output voltage of the fuel cell in that type of power generator system.
2. Description of the Prior Art
The output voltage of a fuel cell may typically have its value varying greatly, depending upon whether the fuel cell is actually supplying its power to any particular load or not.
It is also known that the fuel cell will degrade if it is left in its high voltage level (such as above 0.8 Volts per cell), and it will also degrade due to the reversed polarity effect that may occur if the fuel cell is at its low voltage level.
The power generator system that employs the fuel cell converts a DC electrical power supplied by the fuel cell into a DC power or AC power by using any appropriate DC-to-DC converter or DC-to-AC converter, so that either of the converted currents can be supplied to its attached load. As the output voltage of the fuel cell may vary greatly as described above, the DC-to-DC converted or DC-to-AC converter includes a power element which usually have a high withstand voltage property.
When the fuel cell is left in its high voltage state, an electrochemical reaction may be caused by any residual gases that remain in the fuel cell if the power generator system is stopped (therefore the converter is also stopped). This reaction produces a high voltage when the system is placed under no load condition. In the prior art, a discharging circuit is incorporated in the power generator system to remove the high voltage as soon as it is produced by the reaction.
When the discharging circuit is used to remove the high voltage, it must be controlled by its own power supply, and this control power supply is typically provided from any external power source. It should be noted, however, that for a mobile fuel-cell power generator system, it would be practically difficult to use the external power source as such control power supply.
The power generator system that employs the fuel cell (which will be referred to hereinafter as "FC") may typically be started up or stopped in the manner as shown in FIG. 1. The curves 401, 402, and 403 shown in FIG. 1 represent the respective changes over a given time in the FC voltage, the FC current, and the amount of the FC introduced gas. The FC rising temperature period (t1) corresponds to the period from the moment that the FC1 is started up until the moment that any FC gas, such as fuel gas and oxygen gas is to be fed into FC1. Once any FC gas is fed into the FC1, the FC voltage is increasing rapidly. The FC open-circuit voltage checking period (t.sub.2) corresponds to the period from the moment that the gas has been fed into the FC1 until the moment that the FC1 generates electricity and is beginning to supply the generated electricity.
The generated power increasing period (t.sub.3) corresponds to the transitional period from the start of the power supply until the moment that the power supply can become constant. During the following period (t.sub.4) when the particular power requirements may be supplied, the power will be provided from the FC1 in the constant manner. The final-stage period (t.sub.5) corresponds to the time that may be required from the moment that the FC gas has ceased to be fed, causing the FC1 to stop its power supply until the moment that the FC voltage is falling to approximately zero (0).
Those segments designated by A, C, and b, respectively, in FIG. 1 will be described later by referring to FIGS. 4 and 5.
The sequence of starting up and stopping the known fuel-cell power generator system as described above and shown in FIG. 1 may be used in the plant facilities, regardless of their running capacities. Specifically, the sequence of starting up may be found in the publication "FUJI JIHOU Vol. 61, No. 2, 1988, 156 (40) page, Japan". The following describes some of the prior art sequences of producing a DC power in the fuel cell and converting the DC power into the required power to be supplied to any particular load.
The sequence that is implemented by the circuit shown in FIG. 2 consists of allowing FC1 to provide a DC power output and then allowing an invertor (INV) 2 to convert the DC power output to the corresponding AC power which is to be supplied to a particular AC load 3 or other operational systems 4.
The AC power output of the invertor 2 is delivered to a filter (FL) 5 and then to a transformer (TR) 6, from which the AC power will be supplied to the AC load 3 or other operating systems 4 when an output switch (SW.sub.AC) 7 is placed in its "ON" position. In FIG. 2, reference numeral 8 refers to a main switch (SW.sub.M), 9 refers to a discharging resistance switch (SW.sub.R), and 10 refers to a discharging resistor (RD).
The circuit shown in FIGS. 3A and 3B includes a step-up chopper 11 that provides the DC-to-DC conversions in response to the DC power output from the FC1. Those parts or elements which are similar to those shown in FIG. 2 are given the same reference numerals in FIGS. 3A and 3B. Specifically, the circuit arrangement shown in FIG. 3A includes a backup battery (B) 15 which permits the fuel cell power generator system to be operational in the hybrid manner and to provide a DC power to any particular DC load 3A.
FIG. 3B illustrates a circuit arrangement that includes the step-up chopper 11 of FIG. 3A that provides the DC-to-DC conversions and an invertor 2A that provides an AC power in response to the DC power output of the step-up chopper 11 of FIG. 3A and a filter 5A that provides an AC output to the AC load 3.
FIG. 4 shows the characteristic curves for the fuel cell shown in FIG. 2. In FIG. 4, a curve 701 represents the relationships between the FC current I.sub.F and the FC voltage V.sub.P. A curve 702 represents a 100% load resistance line, which may be obtained by connecting the origin and the rated point N at the time when an FC rated current value b is given along the curve 701. The value of the FC voltage that may then be developed is represented by C.
A line 703 represents changes in the discharging resistance, which may be drawn by connection the origin and the point N' at the time when the FC1, the SW.sub.R 9, and the RD10 forms a closed circuit. At point N', an FC current value a is provided, A line 704 represents a resistance line when the FC current value a' is given along the curve 701. At the point N', FC voltage B' may be developed.
As it may be seen from FIG. 1, the invertor 2 must be able to start at the moment when the power is to be supplied, and must have the ability to provide a current flow equal to the current value b during the period (t.sub.4) from the start-up time to stop time. In other words, the invertor 2 must meet the requirements for the capacity, which may be represented by the area delimited by the original and the points A, O, and b in FIG. 4.
FIG. 5 shows the characteristic curves for the fuel cell shown in FIGS. 3A and 3B. Those lines or curves which are similar to those shown in FIG. 4 are given the same reference numbers in FIG. 5. In FIG. 5, reference numbers 801, 802, 803, and 804 correspond to the lines or curves 701, 702, 703, and 704, respectively. The curve 805 explains the relationships between the battery current I.sub.B and the battery voltage V.sub.B.
As it may be seen from FIG. 1, the FC voltage is gradually increasing during the period t.sub.2 until it has become higher than the battery voltage F as shown in FIG. 5 when the step-up chopper 11 has stopped. Although the step-up chopper 11 is inoperational during that period, a current can still flow through the FC1, a coil L, and a diode D into the battery 15, or the load 3A or 3. The step-up chopper 11 should start at the time when power is to be supplied as shown in FIG. 1, and must meet the requirements for the capacity, which may be represented by the area either delimited by the origin and the points A, O and b, or the origin and the points F, O" and b in FIG. 5.
At the start-up of the fuel cell, the open-circuit voltage checking period (t.sub.2) in FIG. 1 corresponds to the period during which checking is made to see whether any problem has occurred within the FC gas supply system by determining that the FC generated voltage has increased above the prescribed voltage value. During this stage, any means should be provided to allow such checking to occur under no load condition in order to determine whether there is any problem with the FC gas supply system.
If a problem has occurred within the FC gas supply system, the FC generated voltage will fall. In this case, if power is supplied to the load 3 or 3A, the FC generated voltage will fall. In this situation, it would disadvantageously be impossible to check to see accurately if there is any problem with the FC1.
As it may be understood from FIG. 1, when the system is stopped, the FC gas supply will also be shut down. In this case, some of the FC gas may possibly remain within the FC1 and its associated conduit passages, causing some voltage to be developed. Therefore, means such as the open-circuit capacity converter should be provided in the FC1.
Another disadvantage of the prior art is the use of the power element that has a high withstand voltage property as one of the component parts of the converter. Using such power element increases the physical dimensions of the converter, and reduces its efficiency of conversion.